Symbiosis of [18F]FDG-PET/CT and [68Ga]DOTATATE-PET/MRI in differentiated thyroid cancer: Towards a more holistic approach? Preliminary results of a single centre study

Objectives To evaluate whether [68Ga]DOTATATE-PET/MRI with diffusion weighted imaging (DWI) can complement or replace [18F]FDG-PET/CT in patients with recurrent radioiodine negative differentiated thyroid cancer (DTC).

Methods 13 thyroidectomised and remnant ablated DTC patients underwent [18F]FDG-PET/CT and [68Ga]DOTATATE-PET/MRI within 8 weeks. Localization of recurrent cancer was evaluated on a per-patient, per-organ (lymph nodes, lung, and bone) and per-lesion basis. Histology, prior and follow-up examinations served as standard of reference.

Results In a patient based analysis all of the 12 patients were correctly identified with [68Ga]DOTATATE-PET/MRI, whereas one patient with local relapse, was missed by [18F]FDG-PET/CT.Overall lesion detection rates were 95/101 (94%), 69/85 (81%) and 27/55 (33%) for [18F]FDG-PET/CT, [68Ga]DOTATATE-PET/MRI and DWI respectively. In the overall lesion evaluation [18F]FDG(-PET/CT) performed better than [68Ga]DOTATATE(-PET/MRI), yet this was statistically significant only when comparing [18F]FDG-PET/CT, with [68Ga]DOTATATE-PET/MRI (p=0.039; [18F]FDG-PET vs. [68Ga]DOTATATE: p=0.062). There was a significant difference between [18F]FDG-PET/CT and [68Ga]DOTATATE-PET/MRI with regards to detection of pulmonary metastasis (p < 0.0001), that was also seen when comparing [18F]FDG-PET and [68Ga]DOTATATE-PET (p=0.001) with each other. Reanalysing the performance of the two hybrid modalities and the two PET-tracers after excluding the pulmonary metastasis, [68Ga]DOTATATE-PET and [68Ga]DOTATATE-PET/MRI showed a superior sensitivity over [18F]FDG-PET (p=0.005) and [18F]FDG-PET/CT (p=0.102) respectively, to the disadvantage of specificity. On the contrary, DWI achieved only poor sensitivity rates and was significantly different from [18F]FDG-PET in the lesion based overall evaluation (p < 0.0001), pulmonary metastasis (p=0.001) and overall evaluation excl. pulmonary metastasis (p < 0.0001). Interestingly, out of the 6 bone metastasis present in 2 patients, [18F]FDG-PET/CT failed to detect 3 of them (detection rate: 50%) , whereas [68Ga]DOTATATE-PET/MRI detected all 6 (100%) and DWI identified correctly 2 of the 4 (50%) bone metastasis that were located supradiaphragmatical.

Conclusions [18F]FDG-PET/CT overall performs better than [68Ga]DOTATATE-PET/MRI in DTC mostly due to the superiority of both CT and [18F]FDG-PET over [68Ga]DOTATATE-PET and MRI respectively, in the detection of lung metastasis. However, in the evaluation of extrapulmonary lesions [68Ga]DOTATATE-PET is the more sensitive and [18F]FDG-PET the more specific imaging modality. Owing to the very low sensitivity we could not demonstrate any added value of DWI in patients with DTC. Therefore, DWI is not essential in clinical PET/MRI protocols and cannot replace [18F]FDG-PET for monitoring postoperative DTC patients with suspected radioiodine-negative tumour burden.